Since seed has a higher value than grain, the standard of management to be applied ought also to be higher. However, in principle, agronomic practices for seed crops are similar to those for normal crops, in that due attention must be given to field practices that improve yield (e.g., optimizing the timing of planting, appropriate and calibrated fertilizer application, conservation agriculture, and water management) and to minimizing factors that reduce yield (e.g., water stress, pests, and diseases). In addition, safety procedures must be followed when using machinery or applying fertilizers and chemicals. In types of hybrid seed production where one or both parents are inbred lines, extra care needs to be taken with those lines, which tend to be weak and more susceptible to environmental stresses (e.g., diseases, certain herbicides, and nutrient stress) than hybrids.
The seed of inbred line parents is usually smaller than the normal maize seed. While seed size does not necessarily affect germination percentage, it may affect the ability to emerge quickly and resultant seedling vigor. Consequently, inbred lines should not be sown too deep or into a cloddy seedbed. If sowing is done by machine, differential calibration of the planting units for the male and female may be necessary, based on required plant density, seed size, and germination percentage of each of the two parental components.
The better the agronomic management applied to a seed field — especially in the timing of operations and the efficiency and appropriateness of fertilizer, herbicide and pesticide application, weed control, and water management — the greater will be the yield potential. However, because these production factors are determined by field context and environment, they will not be discussed in detail. Local expert advice relevant to each seed field should be sought. Here, only issues related to hybrid quality assurance will be discussed, but note the following general principles for good crop management:
Well before planting time, collect soil samples from the seed field and obtain a soil nutrient test report and fertilizer recommendation. With three-way and double-cross hybrids, the females have higher yield potential than the females of single-cross hybrids, so nutrient removal by the females will be greater in seed production fields of three-way and doublecross hybrids. In the case of single-cross hybrid production, the female is an inbred line, which uses less fertilizer but still requires fertile soil to produce well. This also applies to the males of single-cross and three-way hybrids.
If the soil is acidic, apply lime with appropriate calcium and magnesium ratios according to the soil clay content, soil pH, and soil test results. Lime should be applied three to six months before sowing, to have time to take effect (lime applied at sowing will not be effective for the emerging crop).
Apply phosphorous and potassium fertilizers just prior to or at the time of sowing. The effectiveness of these fertilizers is greatest when they are applied in a band alongside and below the seed. Ensure that there is no direct contact between fertilizer and the seed so as to avoid fertilizer burning of the germinating seed. About one-third of the total nitrogen fertilizer should be applied at or before sowing. Inbred lines may be more sensitive to micronutrient deficiencies than hybrids. Observe the growth of inbred lines, look for nutrient deficiency symptoms, and correct these with foliar or soil-applied fertilizers if required. Where known micronutrient deficiencies occur in the soil, correct these with pre-plant fertilizers.
Certain maize herbicides, especially those in the Sulfonylurea and Chloroacetamides groups, maybe phytotoxic to inbred lines. Check with the seed company representatives regarding which herbicides may be applied to the seed parents being grown. When applying post-emergent herbicides, it is advisable to band apply these between the rows, avoiding foliar contact.
Time sowing so that flowering occurs when reliable rains are expected, but also at a time that will avoid the crop maturing during wet weather, so as to avoid rain damage to maturing seed in the field.
The sowing of hybrid seeds is critical—it determines crop establishment and potential. Therefore, sow the two parents at the recommended times, aim for the correct plant density and conduct operations to achieve a uniform emergence. Ensure the micro seedbed is optimal for germination and emergence, considering seed-soil contact, sowing depth, soil moisture conditions, and avoiding compaction.
Beware of soil pests and seedling diseases that may reduce stand establishment and apply recommended seed treatments or pesticides to the soil, if needed.
- Monitor and manage the crop throughout the season, controlling weeds (inbred lines are particularly sensitive to weed competition), pests and diseases, and providing irrigation if possible. In most seed regulations, weedy fields, especially at harvest, will disqualify a seed crop from certification.
- The remaining two-thirds of the nitrogen application may be applied in two side-dressings, one at the 4-5 leaf stage and the other at the 8-10 leaf stage. Use the application method best suited to the type of fertilizer, crop stage, machinery available and weather conditions. Avoid wastage of nitrogen by leaching, run-off or volatilization.
- At flowering time, check for stalk borer and bollworm on the silks of the females, as these pests may feed on the silks and reduce seed set or feed on the developing kernels and reduce seed quality and yield.
- The four-week period spanning the flowering stage of seed maize is most sensitive to water deficits. If a dry spell occurs at this time, irrigation will significantly improve yields.
- In countries where maize is grown on acid soils, it is beneficial to do a foliar application of 100-150g/ha of sodium molybdate at the mid-silk to milk-dough stage of the females. This increases the molybdenum concentration in the seed and helps overcome molybdenum deficiencies in the field.
- Foliar applications in the late vegetative and early grain-fill stages with recommended fungicides for expected diseases will improve yields and seed quality. Apply these chemicals according to label instructions.
- Harvest the seed as early as possible, considering drying facilities and processing requirements. Reducing the time mature seed remains in the field improves seed quality by minimizing pest and disease infestation.
Hybrid seed production is a step-wise process, with each step influencing the following steps. Managing hybrid seed production involves two parallel processes: the technical aspects of the seed crop, such as female-male ratios, detasseling, etc., and the agronomic field management of the female and male plants.
Furthermore, there is a close relationship between the grower of the seed, the seed company for whom the seed is being grown, and the regulatory agency that will certify the seed crop. It is evident therefore that maize seed production is more complex and dynamic than grain production. Success depends on beginning with a thorough understanding of the whole process, planning crop areas, inputs, activities, and labor requirements, maintaining good communications with labor, the seed company, and regulators, and carrying out management activities in a timely and efficient manner. The ultimate goal of hybrid seed production is the high yield of seed that meets quality standards. Only when this is achieved can any thought or hope of profit be considered. Thus, successful hybrid seed production begins with planning the whole process from the beginning to the end. Be certain to match the proposed area of seed production with financial, labor, time, machinery, and management resources.
Selecting the seed field
The best fields on a farm should be used for hybrid seed production, but other considerations include:
- Accessibility – the seed field needs to be inspected regularly and therefore must be easy to reach throughout the season.
- Cropping history – the previous crop should not be maize, to minimize the possibility of re-growth plants in the seed field.
- Isolation – the field must be sufficiently isolated from contaminant maize crops by the required distance or time.
- Size of field – due to the time pressure for detasseling in hybrid seed fields, a maximum field size of 10 to 40 ha should be planted at one time, depending on labor availability. Blocks of about 10 ha are most manageable for detasseling and quality assurance. Smaller fields may increase the risk of foreign pollen contamination. Larger fields may not allow efficient detasseling.
- Field map – this is helpful to establish the size of the field, isolation distances and for future records.
- Registration of the seed crop – usually seed regulation authorities require seed fields to be registered within a short time of establishment. Ensure that this is done.
Handling the parent seed
The parent seed of a hybrid is precious, therefore handle it with care.
- Keep the male and female seed bags separate.
- Be careful not to lose the labels attached to the bags, and retain these after sowing the seed for certification purposes and for any claims that may arise.
- Store seed in a cool, dry place.
- Protect the seed from rodents and storage pests.
- Keep the seed secure from theft.
- Handle seed gently.